This invention relates to a passive infrared intrusion detector (PIR intrusion detector) useful for detecting intruders (objects emanating infrared radiation) trespassing in rooms to be protected. Such a PIR intrusion detector comprises a pyroelectric sensor with several radiation-sensitive sensor elements in close spatial relationship for providing signals to an evaluating circuit in order to produce an alarm signal when the output of the sensor exceeds a predetermined threshold value.
PIR intrusion detectors of this general type, for detecting the intrusion of a person or any object emitting infrared radiation in a supervised area, are known in the art.
Such PIR intrusion detectors in general include a lens or other appropriate focusing element which focuses infrared radiation from fields of view onto infrared radiation-sensitive elements which typically include first and second spaced electrodes between which pyroelectric material is positioned. Each of these elements is operative to produce an electric charge proportional to the rate of change of infrared radiation incident thereon. The elements are spaced from one another. Pyroelectric materials include plastic film materials such as polyvinylidene fluoride, crystal materials such as lithium tantalate, and ceramic materials such as lead zirconate titanate. The pyroelectric material of each element is polarized and the sensor elements form a capacitor with a capacitance typically of an order of magnitude of 10 pF. When radiant infrared energy falls upon the material, the polarization of the material is changed, a small electric charge appears between the electrodes due to the internal transfer of electric charge which generates a voltage across a resistance. This voltage is impedance transformed by a field effect transistor FET. The output voltage of the FET is amplified in an evaluation circuit to signal an intrusion. The amplified signal can be used to produce an alarm or to activate flood lamps and/or other deterrents.
In intrusion detection systems it is possible to use detectors having only a single sensor element consisting of pyroelectric material: preferably however, the sensor comprises two or more sensor elements. Such arrays are used for instance to attain a certain immunity to undesired signals caused by changes of the temperature of the environment or changes of the background radiation, by combining the two sensor elements in a differential circuit. According to one prior embodiment of this intrusion detector the sensor elements are arranged in such manner that uniform changes of the radiation incident thereon, for instance changes of the background radiation, produce compensating charges on both sensor elements which are connected in a circuit differentially with result that no output signal is produced. In case radiation from the area to be protected impinges only on one sensor element, a differential output signal is produced.
In such a known infrared detector, a pair of sensor elements is arranged in a common plane and the sensor elements consist of thin wafers of polarized pyroelectric material. Opposite surfaces of the sensor element electrodes, at least one of which is substantially transparent, are arranged in overlapping manner and the electrodes extend in a direction normal to the direction of the polarization. The sensor elements are interconnected within the sensor by means of their electrodes, effectively resulting in two capacitors connected in series or parallel, the polarity of the pyroelectric material of the two sensors within the series or parallel circuit being opposite to each other. Such sensors are called "dual" sensors.
Dual sensors are very useful in intrusion detection systems. A great advantage of the dual sensors is that fluctuations in the thermal state of the background do not result in a change of the output signal of the intrusion detector. In intrusion detectors comprising single sensor elements, such temperature fluctuations cause a fault signal and as a consequence a false alarm.
In Weitman U.S. Pat. No. 4,614,938 an intrusion detection system with a sensor comprising four or more sensor elements is described. The sensor consists of two dual sensors with two separate evaluation channels in the same housing. The four sensor elements are closely spaced to one another and cover substantially all of at least one surface of a single piece of pyroelectric material. Multiple fields of view of areas under surveillance are covered. An advantage of the infrared intrusion detector system of this patent is that occasional spurious current peaks (or bursts or spikes) in the pyroelectric material caused by large changes in the environmental temperature, which with normal dual sensors would result in a false alarm, are suppressed. Only if in both of the evaluation channels concurrent intruder signals are generated, is an alarm output produced. The probability that an occasional spurious signal of the type mentioned occurring coincidentally in both evaluation channels is comparatively small.
However, the infrared intrusion detector system according to U.S. Pat. No. 4,614,938 has the disadvantage that the sensor elements of the two dual sensors are arranged in a staggered way and, consequently, the two separate evaluation channels supervise non-overlapping areas of sensitivity through a coordinated optical system. Consequently, an intruder must cover a greater distance within a few seconds to produce concurrent signals, in order to be positively detected. A further disadvantage of the infrared intrusion detector system according to this patent is that two dual sensors and two separate evaluation channels are necessary and, consequently, two preamplifiers are needed which raises the price of the detector. Further, the reliability of the detector against false alarms caused by turbulent warm or cold air and by radio frequency interferences is not better than with a normal dual sensor, since these disturbances penetrate both of the evaluation channels.
In Baker U.S. Pat. No. 4,697,081 it is suggested that the disadvantage of the non-overlapping areas of sensitivity and consequent need for an intruder to cover a greater distance within a predetermined time can be overcome, by using a detector wherein one pyroelectric sensor element of one dual sensor is interdigitated with a respective sensor element of the other dual sensor. As a consequence, through the optical system of the detector both evaluation channels supervise practically the same area, the intrusion signal appears without delay in both channels, the distance which an intruder has to cover is reduced, and the detection is more positive. However, the other above-mentioned disadvantages are not affected by this arrangement of the sensor elements.